3.3 - Counting Atoms

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<p> 1. Section 3 Counting Atoms Objectives Explain what isotopes are. Define atomic number and mass number, and describe how they apply to isotopes. Given the identity of a nuclide, determine its number of protons, neutrons, and electrons. Define mole, Avogadros number, and molar mass, and state how all three are related. Chapter 3 Solve problems involving mass in grams, amount in moles, and number of atoms of an element. 2. Section 3 Counting Atoms Atomic Number Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. The atomic number (Z) of an element is the number of protons of each atom of that element. Chapter 3 3. Section 3 Counting Atoms Isotopes Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. Most of the elements consist of mixtures of isotopes. Chapter 3 4. Section 3 Counting Atoms Mass Number The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. Chapter 3 5. Section 3 Counting Atoms Designating Isotopes Hyphen notation: The mass number is written with a hyphen after the name of the element. uranium-235 Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number. 235 92 U Chapter 3 6. Section 3 Counting Atoms Designating Isotopes, continued The number of neutrons is found by subtracting the atomic number from the mass number. mass number atomic number = number of neutrons 235 (protons + neutrons) 92 protons = 143 neutrons Nuclide is a general term for a specific isotope of an element. Chapter 3 7. Section 3 Counting Atoms Designating Isotopes, continued Sample Problem A How many protons, electrons, and neutrons are there in an atom of chlorine-37? Chapter 3 8. Section 3 Counting Atoms Designating Isotopes, continued Sample Problem A Solution Given: name and mass number of chlorine-37 Unknown: numbers of protons, electrons, and neutrons Solution: atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons Chapter 3 9. Section 3 Counting Atoms Designating Isotopes, continued Sample Problem A Solution, continued Chapter 3 mass number of chlorine-37 atomic number of chlorine = number of neutrons in chlorine-37 An atom of chlorine-37 is made up of 17 electrons, 17 protons, and 20 neutrons. mass number atomic number = 37 (protons plus neutrons) 17 protons = 20 neutrons 10. Section 3 Counting Atoms Relating Mass to Numbers of Atoms The Mole Chapter 3 The mole is the SI unit for amount of substance. A mole (abbreviated mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. Avogadros Number Avogadros number6.022 1415 1023 is the number of particles in exactly one mole of a pure substance. 11. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Molar Mass Chapter 3 The mass of one mole of a pure substance is called the molar mass of that substance. Molar mass is usually written in units of g/mol. The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units. 12. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Gram/Mole Conversions 4.00 g He 2.00 mol He = 8.00 g He 1 mol He Chapter 3 Chemists use molar mass as a conversion factor in chemical calculations. For example, the molar mass of helium is 4.00 g He/mol He. To find how many grams of helium there are in two moles of helium, multiply by the molar mass. 13. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Conversions with Avogadros Number Avogadros number can be used to find the number of atoms of an element from the amount in moles or to find the amount of an element in moles from the number of atoms. In these calculations, Avogadros number is expressed in units of atoms per mole. Chapter 3 14. Solving Mole Problems Section 3 Counting Atoms Chapter 3 15. Determining the Mass from the Amount in Moles Section 3 Counting Atoms Chapter 3 16. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B What is the mass in grams of 3.50 mol of the element copper, Cu? Chapter 3 17. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B Solution Given: 3.50 mol Cu Unknown: mass of Cu in grams Solution: the mass of an element in grams can be calculated by multiplying the amount of the element in moles by the elements molar mass. grams Cu moles Cu = grams Cu moles Cu Chapter 3 18. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B Solution, continued The molar mass of copper from the periodic table is rounded to 63.55 g/mol. 63.55 g Cu 3.50 mol Cu = 1 222 mol Cu g Cu Chapter 3 19. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem C A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced? Chapter 3 20. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem C Solution Given: 11.9 g Al Unknown: amount of Al in moles Solution: moles Al grams Al = moles Al grams Al 1 mol Al 11.9 g Al = 26. 0.441 98 g Al mol Al Chapter 3 The molar mass of aluminum from the periodic table is rounded to 26.98 g/mol. 21. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem D How many moles of silver, Ag, are in 3.01 1023 atoms of silver? Chapter 3 22. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem D Solution Given: 3.01 1023 atoms of Ag Unknown: amount of Ag in moles Solution: moles Ag Ag atoms = moles Ag Avogadro's number of Ag atoms 23 23 1 mol Ag 3.01 10 Ag atoms 6.022 10 Ag at 0.500 = m oms ol Ag Chapter 3 23. Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem E What is the mass in grams of 1.20 108 atoms of copper, Cu? Chapter 3 24. Section 3 Counting Atoms Sample Problem E Solution Given: 1.20 108 atoms of Cu Unknown: mass of Cu in grams Solution: moles Cu grams Cu Cu atoms = grams Cu Avogadro's number of Cu atoms moles Cu 14 8 23 1 mol Cu 63.55 g Cu 1.20 10 Cu atoms = 6.022 10 Cu atoms 1 mol Cu 1.27 10 Cug Relating Mass to Numbers of Atoms, continued Chapter 3 The molar mass of copper from the periodic table is rounded to 63.55 g/mol. </p>